WASHINGTON – Iran announced that it resumed work at a key nuclear facility yesterday, effectively ending a suspension of its nuclear program and severing more than two years of negotiations with Europe that held out the promise of closer ties with the West.

The Bush administration, along with allies Britain, France, and Germany, had warned they would try to take the issue of Iran’s program to the U.N. Security Council if Tehran made good, as it did yesterday, on promises to restart a uranium conversion facility in the town of Isfahan.

An emergency meeting of the 35-member board of the International Atomic Energy Agency was set for today in Vienna to discuss the latest Iranian moves. But it was unlikely the board would immediately refer the matter to the Security Council because Iran’s actions do not break any international laws.

Iran agreed in November, after months of talks with the three European countries, to suspend its nuclear program while the four parties discussed the possibility of a final agreement that would bring about an end to suspicions about Tehran’s nuclear ambitions.

On Friday, the European trio offered Iran a package of incentives in exchange for a legally binding commitment by Iran to permanently forgo much of its nuclear program. Iran, which claims to be exercising its legal rights to a nuclear energy program under the Nuclear Nonproliferation Treaty, has said it would not give up the program.

It formally rejected the European offer yesterday, calling the proposal a “clear violation of international law” that seeks to force Iran into accepting “intrusive and illegal inspections,” according to a copy of the response, which was made available by a senior Iranian official.

The response said the European plan offers “no firm guarantees or commitments” about future security, technology, and economic cooperation, and demands that Iran abandon most of its peaceful nuclear program.

An Iranian official involved in the negotiations called the proposal “very bad. I read it trying to find anything that you could present as meaningful, and it’s really insulting.”

The official response also termed the European offer an insult and said the three European nations must apologize for it. The response, relayed today to the Europeans, called their offer a “mockery” of the Paris agreement reached last November.

Iran has maintained that its program, built in secret over 18 years, is designed to bring the country a new energy source, and is not for nuclear weapons. But the scale of the effort and its clandestine nature has fueled long-held suspicions in Washington that Iran intends to build atomic weapons.

At the conversion facility, 10 miles southeast of the city of Isfahan, Iranian scientists carried out an early stage of the nuclear fuel cycle yesterday, in which raw uranium is dissolved into a solution that can be further converted.

IAEA officials have said that an area of the facility now in use is for the first of three phases of conversion, a lengthy and technical process that could eventually yield bomb grade uranium.

The official Islamic Republic News Agency announced that the Isfahan plant resumed work yesterday with uranium ore “taken into a special room for injection, sampling, and other reprocessing activities.” According to the announcement, the plant will soon begin operating the next two phases in the conversion process.